mmc: usdhi6rol0: handle NULL data in timeout

Commit bb08a7d4 ("mmc: usdhi6rol0: fix NULL pointer deref in debug
print") fixed one NULL pointer dereference but unfortunately introduced
another.  "data" may be NULL if this is a command timeout for a command
without any data, so we should only use it if we're actually waiting for
data.

Fixes: bb08a7d4 ("mmc: usdhi6rol0: fix NULL pointer deref in debug print")
Cc: stable@vger.kernel.org # v4.3
Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>